my power supply is 400w, do I need a 400w UPS?

Not necessarily as your system isn't using 400W, that just means your psu could support UP TO 400W. And that's the same case with a UPS. 400W means UP TO 400W. Even though your system is maybe using around 200W, the higher the watts the UPS is rated at, the longer it would be able to run your system without power. So a 400W UPS would generally run your system longer than a 200W UPS. But other than that, you'd just have to read into a specific UPS to see how long it's expected to last @ a certain number of Watts and so on.

jAce

Posted 2014-08-17T18:07:39.817

Reputation: 1 222

Bigger UPS will last longer though – pqnet – 2014-08-17T18:27:10.830

Most UPS are rated in VA, not watts. Also, it usually states on the box what you can expect from the UPS. – LDC3 – 2014-08-17T18:32:58.360

@LDC3: A volt-amp is a synonym for a watt. – Ben Crowell – 2014-08-17T18:42:50.403

P=IV Power=Amperage/Voltage Dont remember why "I" stands for amps... – Keltari – 2014-08-17T18:53:03.540

1"I" stands for "intensity of an electrical current, or actually the french word for intensity. – jAce – 2014-08-17T18:56:25.797

@BenCrowell This 600VA UPS is also rated at 360W. I don't know why there is a difference. http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=8034506&CatId=233

– LDC3 – 2014-08-17T19:08:44.060

VA == W, but there is a important distinction in usage - Watts are raw, VAs "apparent".
If you have a 400W PSU and it has a common PFC efficiency rating of 80(%), it actually feeds 500 VA from the grid to produce the 400 W for your appliance (you get charged for 500W)

You have to be below both ratings. - so the 600/360 UPS can handle a 360W PSU with efficency of 60% or more. The reason VA is advertised more is that it's a "dark pattern" - a higher number. If you have a more effective PSU it doesn't matter, the UPS wont allow more watts drawn. – rogerovo – 2014-08-17T19:43:08.020

Even the cheapest PSUs have a rating of 70+%, so you actually can ignore VA's - if not stated on the UPS, apply the "industry standard" VAx0.6 to check the max. PSU-Watts – rogerovo – 2014-08-17T19:50:00.320

Also, in the world of AC power, VA means the RMS voltage multiplied by the RMS current. When there is a phase-shift between the voltage and the current (which happens when the load is inductive e.g. an electric motor), the RMS power is different from the VA. In fact, in parts of the AC cycle, the power can go negative! – Adrian Pronk – 2014-08-18T05:24:18.683

my power supply is 400w, do I need a 400w UPS?

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